It is known to produce chlorine dioxide by reduction of chlorate ions with chloride ions in the aqueous phase in the presence of free hydrogen ions, in accordance with the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
This process may be effected in a number of ways, broadly falling into two groups. In the first group, the chloride ions are added to the reaction medium, such as, in the form of a chloride salt, usually an alkali metal chloride, preferably sodium chloride, or as hydrochloric acid, while, in the second group, the chloride ions are formed in situ by reduction of the chlorine, usually using sulphur dioxide or methanol as the reducing agent.
A variety of strong acids may be used, alone or in admixture, to provide the free hydrogen ions required in the chlorine dioxide-producing reaction, such as, sulphuric acid, hydrochloric acid and phosphoric acid. Where hydrochloric acid alone is used as the source of the free hydrogen ions, it also acts as the source of chloride ion reducing agent. Where hydrochloric acid is used in admixture with other acids, it may also provide part or all of the chloride ion reducing agent, depending on the molar quantity used.
The chlorate ions usually are introduced to the reaction medium in the form of an alkali metal salt, preferably sodium chlorate. The cation of the chlorate, along with any other cations introduced to the reaction medium, combines with the anion of the acid to form a by-product salt. The following equations illustrate the formation of these by-products: EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.Cl0.sub.2 +1/2Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4 EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+NaCl EQU 2NaClO.sub.3 +SO.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2ClO.sub.2 +2NaHSO.sub.4
The by-product is removed from the reaction medium, on a continuous or intermittent basis, by crystallization as a solid phase. Such crystallization may be effected inside or outside the reaction vessel. Removal of the crystalline material from the mother liquor usually results in entrainment of some mother liquor.
Conventional separation techniques, such as, filtration and centrifugal separation, often in combination with wash water, have been used to separate the mother liquor and purify the by-product crystals, the separated mother liquor and spent wash water usually being returned to the generator containing the reaction medium to avoid loss of the chemical values thereof.